Investigating Electrode Reactions in Vanadium Redox Flow Batteries - a Distribution of Relaxation Times Analysis

Due to the increasing energy demand worldwide and the urgency of action because of climate change, new energy storage devices are needed to balance the fluctuations of renewable energy sources. The Vanadium Redox Flow Battery (VRFB) is a promising technology for large-scale energy storage but still needs to overcome significant lifetime and efficiency challenges, which are decreased by polarization losses during operation. It is essential to conduct experiments in a setup that closely mimics the cell's operating conditions to analyze the ongoing processes in a VRFB. Therefore, setups reflecting a half cell are well-suited. Thus, new insights into the reaction and transport processes in a VRFB can be gained. Electrochemical Impedance Spectroscopy (EIS) combined with Distribution of Relaxation Times (DRT) analysis is a well-suited method to investigate electrode reactions. This combination of methods is frequently used in research for lithium-ion batteries or in our group for fuel cell characterization (1-3). To the best of our knowledge, there is no study using EIS and DRT analysis to analyze processes in a model half cell in VRFB research published yet, and just very few studies on characterization in full cell-like setups are available (4). Here, we present a novel 3D printed flow cell for investigations under application-oriented conditions, designed to ensure steady-state conditions during the measurements. These studies focus on the characterization of the processes in the positive half cell by EIS to deepen the knowledge about the polarization losses in VRFBs. The vanadium-containing electrolyte is continuously pumped through the cell during the experiments to guarantee consistent conditions while EIS measurements are performed. The electrolyte flows through the carbon paper stack used as electrode material, undergoing an electrochemical reaction. Here, either the oxidation of vanadium(IV) to vanadium(V) or the reduction from vanadium(V) to vanadium(IV) occurs. The recorded data were analyzed with the DRT method, which allows the separation of physicochemical processes on different time scales. Parameters like the temperature, the flow rate, the electrolyte concentration, and the electrolyte species were varied independently to identify the individual processes in the positive half cell of a VRFB. This setup enables electrochemical impedance measurements of high quality, which is essential for a reliable DRT analysis. We could assign the peaks in the DRT spectrum to the electrochemical reaction, the convective transport through the electrode structure, and the diffusion processes of the vanadium species. Thus, we could identify the individual processes in the positive half cell of the VRFB and their contributions to the overall impedance. This information is vital in search of optimized operating conditions with reduced polarization losses. 1. M. A. Danzer, Batteries, 5 (3), 53 (2019). 2. A. Weiß, S. Schindler, S. Galbiati, M. A. Danzer and R. Zeis, Electrochimica Acta, 230 , 391–398 (2017). 3. N. Bevilacqua, M. A. Schmid and R. Zeis, Journal of Power Sources, 471, 228469 (2020). 4. J. Schneider, T. Tichter, P. Khadke, R. Zeis and C. Roth, Electrochimica Acta, 336 , 135510 (2020). Figure 1